Method for annealing of strip coils

ABSTRACT

A strip coil is uncoiled, heated to a predetermined temperature and coiled again. The heated and coiled strip is maintained at a predetermined temperature for a predetermined time and thereafter gradually cooled.

United States Patent 1191 Hirata METHOD FOR ANNEALING OF STRIP COILS[75] Inventor:

[73] Assignee: lshikawajima-Harima Jukogyo Kabushiki Kaisha, Tokyo,Japan 22 Filed: Apr. 20, 1973 21 Appl. No 352,958

Hiromasa Hirata, Yokohama, Japan 521 US. Cl. 148/156; 148/11.5; 148/12;

148/l2.7; 148/13; 148/121 511 1m.c1. ..c211) 9 52 58 Field of Search148/156, 122, 121, 12,

[56] References Cited UNITED STATES PATENTS 2,209,686 7/1940 Crafts1423/12 1451 Sept. 30, 1975 Primary Examiner-Walter R. SatterfieldAttorney, Agent, or Firm-Scrivener Parker Scrivener & Clarke [5 7ABSTRACT A strip coil is uncoiled, heated to a predetermined temperatureand coiled again. The heated and coiled strip is maintained at apredetermined temperature for a predetermined time and thereaftergradually cooled.

1 Claim, 1 Drawing Figure US. Patent Sept. 30,1975

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METHOD FOR ANNEALING OF STRIP COILS The present invention relates to amethod for annealing cold-rolled strip coils.

The batch type annealing method has been generally employed in the coldrolling mill factories. According to this method the upended three orfour strip coils are stacked one upon another and covered with an innercover and a cylindrical furnace body for annealing. Since the coldroller strip coil is coiled very tight, it must be gradually heated fromthe outer periphery thereof until the temperature at the center of thecoil rises to a predetermined temperature (about 720C), and bemaintained at a predetermined temperature for a predetermined time(about 1.5 to 2 hours). Thereafter the strip coil must be graduallycooled.

Therefore the heat cycle becomes extremely long (for about four or fivedays) so that the productivity is low. Furthermore when the strip coilis rapidly heated and then cooled, it is subjected to deformations sothat the operation in the next stage is adversely affected. Moreoversticking occurs due to the thermal expansion of the strip coil and thequality of the strip is deteriorated. (Sticking refers to the phenomenonthat the adjacent strips adhere to each other due to the thermalexpansion.)

In order to overcome the above and other problems there has beenproposed an improved method in which the open-coiled strip is annealedin a batch type annealing furnace such as before mentioned tightfurnace. The open-coiled strip is uniformly and rapidly heated by theconvection of the heated atmosphere gases in the annealing furnace. Butwhen the open-coiled strip is heated too rapidly, it is subjected todeformations by ununiformity heating, and the sufficiently quick heatingof the strip cannot be 'attained even by the convection of the hightemperature atmosphere gases. Furthermore this method has a defect thatthe installation cost is extremely high.

There has been also used a continuous annealing line as the method forannealing of especially thin steel sheet such as tin plate which willnot require the complete annealing. According to this method the stripis continuously uncoiled and passed through a furnace so that it may beheated and maintained at a predetermined temperature for a predeterminedtime. Thereafter the strip is gradually cooled. But this method has adistinct defect that the length of the annealing furnace must beconsiderably increased in order to store the strip in the furnace, whichis transported at high speed, at a predetermined temperature for apredetermined time (generally 1.5 to 2 hours) required for thesuffilcient grain growth in the strip. (Grain growth refers to aphenomenon that a nucleus of crystal is formed and grown in thestructure of steel. The larger the grain growth, the better the effectof annealing becomes). in practice the strip is maintained at apredetermined temperature only for 30 to 60 seconds so that the annealedstrip is hard and is used only for the manufacture of tin plate. Thatis, the continuous annealing method of the type described thereforecannot be used for producing complete annealing and drawing quality ofcold rolled strips.

The cooling rate is the most important factor in annealing, and it ispreferable to cool the strip at as slow cooling rate as possible fromthe metallurgical viewpoint. However, in practice rapid cooling isgenerally employed in order to improve the production efficiency.Therefore the strip is cooled quickly from a temperature of about 500Cbelow a critical range. (At a higher temperataure all of the carbon insteel exists in the form of saturated solid solution, and when steel iscooled rapidly carbon is not precipitated along the equilibrium line inthe phase diagram so that the oversaturated solid solution of carbonremains in the ferrite. The tendency of the excess carbon to precipitateas carbide thereby recovering the equilibrium is called age hardening.The slow cooling temperature range in which no age hardening occurs iscalled a critical range.)

The present invention therefore provides a method for annealing of stripcoils which utilizes the advantages of the prior art methods andapparatus and may provide high-quality and drawing-quality strip coilsin a highly effective productivity. Briefly stated the present inventionis characterized in that a strip is heated, coiled so as to keep heatand then gradually cooled.

The present invention will become more apparent from the followingdescription of one preferred embodiment thereof taken in conjunctionwith the accompanying drawing in which:

Single FIGURE illustrates a schematic diagram used for the explanationof the annealing method in accordance with the present invention.

in the single FIGURE, reference numeral 1 designates unwinding devices;2, a welder; 3, an electrolytic cleaning line; 4, a loop car fortransporting a strip continuously in a furance area at a predeterminedspeed in order to prevent from the overheat even when the uncoiling lineis stopped while coil preparation and welding; 5, a heating furnace; 6,a nonoxidizing gas purging chamber; 7, rewinders disposed within thepurging chamber 6; 8, upenders; 9, a uniform heating furnace; 10, acooling furnace; 11, strip coils; 12, coil cars; 13, a nonoxidizing gaspurging chamber in which are disposed a unwinder 14, a shear 15 and awelder 16; 17, a jet cooling furnace; 18, a rewinder; 19, strips; 20, atemper mill; 21 and 22, shears; and 23, a flying shear disposed withinthe gas purging chamber 6.

The strips 19 continuously uncoiled from the unwinding devices 1 are fedinto the shears 22 so that the off-gage portions at the leading edges ofthe strips 19 are cut off. Thereafter the strips 19 are formed by thewelder 2 into the form of a continuous strip and fed into theelectrolytic cleaning line 3, then into the loop car 4 and into thecontinuous heating furnace 5 where the strip is heated at a temperatureabout 700-720C for 20 30 seconds. The heated strip 19 is coiled again bythe rewinders 7 in the nonoxidizing purging chamber 6 under thecondition that the strip is heated. Since the strip is separated againby the high-speed flying shear 23 when, the strip is coiled up again, sothat it is not required to stop the heating process. Heat must be alwayssupplied to the heating furnace 5 and the nonoxidizing gas purgingchamber 6 in order to maintain the temperature of the strip 19 at apredetermined constant temperature. The recoiled strip is upended by theupender 8 within the furnace and transported into the uniform heatingfurnace 9 by a suitable means such as a conveyor or coil car. Within theuniform heating furnce 9 the coil is slowly transported by the coil car12 or by a walking beam system etc., so that it may remain within theuniform heating furnace 9 for a sufficient grain growth time 1.5 2hours). Thereafter the strip the rapid cooling process by suitable'meanssuch as a conveyor or coil car, downed and mounted on the unwinder 14within the nonoxidizing gas purging chamber 13 so that the strip coil 11may be uncoiled and passed through the shear 15 and the welder 16 in theform of a continuous strip. Thereafter the continuous strip is cooledrapidly in the jet cooling furnace, and the annealed strip in coiledagain by the rewinding device 18 through the shear 21. Thus the stripannealing process is completed in high productivity.

Unlike the heating furnace, the jet cooling furnace 13 will not beprovided with a loop car because the operation of the jet coolingfurnace can be stopped while the strip coil is mounted upon the unwinder14, welded, thread, and removed from the rewinding device 18. The stripcoil 11 mounted on the unwinder 14 is relatively loose because the coilhas been cooled in the cooling furnace and therefore contracted so thatthe strip coil must be uncoiled without exerting the tension thereto inorder to prevent the strip surface from slippage of each other wrap ofcoil.

The present invention is not limited to the preferred embodimentdescribed hereinafter with reference to the accompanying drawing. Forexample in the embodiment described above the strip coils have beendescribed as being upended by the upender 8, but it will be understoodthat the strip coils may be downended. It is also understood thatvarious modifications and variations can be effected without departingfrom the true spirit of the present invention.

According to the present invention the coiled strip is heated in theform of a continuously uncoiled strip so that the strip may be rapidlyand uniformly heated, maintained at a predetermined temperature for are- "4 quiredtimejandlgradually.cooled. Therefore the --apparatu'srequired for carrying out the annealing method in accord with thepresent invention may be made smaller and the strip may be maintained ata. predetermined temperature for a long time so that the completeannealing maybe effected. Furthermore the strip which has been slowlycooled to a certain temperature is rapidly cooled so that the overallcooling time may be reduced. So not only uniform cooling may beeffected, but also the productivity may be increased. In theconventional cold rolling mill factory, the electrolytic cleaning lineand the temper mill are installed independently, but according to thepresent invention the electrolytic cleaning line 3 is installed at theentrance of the heating line whereas the temper mill 20 is installed atthe exit of the rapid cooling line thereof so that a continuous coldrolling mill factory may be realized. Therefore the general conceptionof a cold rolling mill factory is changed, and the labor saving, thehigh production efficiency, and the reduction in installation cost andin operation cost may be attained.

What is claimed is:

1. A method for continuously. annealing steel strip coils comprising thesteps of a. uncoiling a strip coil,

b. heating the uncoiled strip to an elevated temperature of from 700C to720C for a time period in the range of 20 to 30 seconds,

c. recoiling the heated strip,

d. maintaining the recoiled strip at substantially said elevatedtemperature for a time period in the range of 1.5 to 2 hours,

e. cooling the heated coil for a time period in the-

1. A METHOD FOR CONTINUSOUSLY ANNEALING STEEL STRIP COILS COMPRISING THESTEPS OF A. UNCOILLING A STRIP SOIL, B. HEATING THE UNCOILED STRIP TO ANELEVATED TEMPERATURES OF FROM 700*C TO 720*C FOR A TIME PERIOD IN THERANGE OF 20 TO 30 SECONDS, C. RECOILING THE HATED STRIP. D. MAINTAININGTHE RECOILED STRIP AT SUBSTANTIALLY SAID ELEVATED TEMPERATURE FOR A TIMEPERIOD IN THE RANGE OF 1.5 TO 2 HOURS, E. COOLING THE HEATED COIL FOR ATIME PERIOD IN THE RANGE OF 4 HOURS, F. UNCOILING THE COOLED STRIP ANDRAPIDLY COOLING THE STRIP, AND G. RECOILING THE RAPIDLY COOLED STRIP.